Field of the Invention
The present invention relates generally to an ultrasonic system for detecting the presence of air in a fluid line, and more particularly to an innovative construction used to attach electrical connectors to the ultrasonic transducers using conductive transfer tape to attach the conductive pads of a flex circuit to the thin layer of conductive metal located on each side of the ultrasonic transducer.
In the past there have been two primary techniques which have been used to deliver drugs which may not be orally ingested to a patient. The first such technique is through an injection, or shot, using a syringe and needle which delivers a large dosage at relatively infrequent intervals to the patient. This technique is not always satisfactory, particularly when the drug being administered is potentially lethal, has negative side effects when delivered in a large dosage, or must be delivered more or less continuously to achieve the desired therapeutic effect. This problem results in smaller injections being given at more frequent intervals, a compromise approach not yielding satisfactory results.
Alternatively, the second technique involves administering a continuous flow of medication to the patient, typically through an IV bottle. Medication may also be delivered through an IV system with an injection being made into a complex maze of IV tubes, hoses, and other paraphernalia. With drop counters being used to meter the amount of bulk fluid delivered, many medications still end up being administered in a large dosage through an injection into the IV lines, although the medications may be diluted somewhat by the bulk fluid.
As an alternative to these two techniques of administering medication to a patient, the relatively recent addition of medication infusion pumps has come as a welcome improvement. Medication infusion pumps are utilized to administer drugs to a patient in small, metered doses at frequent intervals or, alternatively, in the case of some devices, at a low but essentially continuous rate. Infusion pump therapy may be electronically controlled to deliver precise, metered doses at exactly determined intervals, thereby providing a beneficial gradual infusion of medication to the patient. In this manner, the infusion pump is able to mimic the natural process whereby chemical balances are maintained more precisely by operating on a continuous time basis.
One of the requirements of a medication infusion system is dictated by the important design consideration of disposability. Since the portion of the device through which medication is pumped must be sterile, in most applications of modern medication infusion equipment some portions of the equipment are used only once and then disposed of, typically at regular intervals such as once daily. It is therefore desirable that the fluid pump portion of the infusion pump device be disposable, with the fluid pump being designed as an attachable cassette which is of inexpensive design, and which is easily installable onto the main pump unit.
It will be perceived that it is desirable to have a simple disposable cassette design to minimize the cost of construction of the cassette, using the minimum number of parts necessary in the design of the cassette. The design of the cassette must be mass producible, and yet result in a uniform cassette which is capable of delivering liquid medication or other therapeutic fluids with a high degree of accuracy. The cassette should include therein more than just a fluid pump; other features which have formerly been included in peripheral devices may be included in the cassette.
Such a system has been disclosed in all of the above-identified previously filed related applications. Of these applications, U.S. Ser. No. 128,121, entitled "Air-In-Line Detector for a Medication Infusion System," is hereby incorporated herein by reference.
An essential function of a medication infusion system is to avoid the infusion of fluid containing more than a minimal amount of air bubbles therein. Although steps may be taken to minimize the possibility of air bubbles being contained in a fluid which is to be infused to a patient, it is essential to monitor the fluid line before it reaches the patient to ensure that air bubbles remain in the fluid which is to be infused are detected. The detection of air bubbles in all fluids which are to be infused is therefore a critical design requirement.
One type of air-in-line detector which has been used in the past is an ultrasonic detector, which places an ultrasonic transmitter on one side of a fluid line and an ultrasonic receiver on the other side of the fluid line Fluid is a good conductor of ultrasonic energy while air or foam is not. Accordingly, if there is an air bubble in the fluid line between the transmitter and the receiver, the signal strength will be greatly attenuated, and the presence of the bubble will be indicated. Examples of ultrasonic air-in-line detectors include U.S. Pat. No. 4,764,166, to Spani, and U.S. Pat. No. 4,821,558, to Pastrone et al.
Typically, the ultrasonic transducers used in an ultrasonic air-in-line detector are made from a ceramic material which is coated on both sides with a thin layer of conductive metal. An electrical connection must be made to each surface of the ultrasonic transducer in order to operate the transducer. The method currently used to make an electrical connection is to solder a wire to the coated thin layer of conductive metal on each side of the ultrasonic transducer.
The technique of soldering has several problems associated with it which adversely affect the performance of the ultrasonic transducer. Typically, the soldering operation will affect the resonant frequency, the Q factor, as well as the useful power output of the ultrasonic transducer. Heat stressing caused by the soldering operation can depolarize the transducer, with the extent of depolarization depending upon the soldering temperature of the soldering iron and the duration of contact.
In addition, the solder adds an additional and highly significant mass to the ultrasonic transducer. This additional mass will likely affect the resonant properties of the ultrasonic transducer to a significant degree. Furthermore, the mass of the solder joint can direct energy away from the sensing interface, thus considerably reducing the output of the ultrasonic transducer. Thus it will be appreciated that the soldering operation presents a number of unfortunate negative aspects which detract from the operation of the ultrasonic sensor.
These disadvantages are further compounded when the process is carried out on a mass production assembly line. Due to assembly error and the difficulty of process control, the soldering process is difficult to administer in a meaningful manner. The net result of the operation will likely be inconsistent ultrasonic transducers, none of which perform up to specifications and many of which must be scrapped.
It is therefore the primary objective of the present invention to provide an improved electrical interface for use with an ultrasonic transducer. The improved interface must have an entirely cold assembly process, so that no heat is required to join the conductors to the thin layers of conductive metal on the sides of the ultrasonic transducer. The resulting electrical connection must be as good as the soldered joint, and must present excellent lifetime characteristics.
The joint between the conductors and the thin layers of conductive metal on the sides of the ultrasonic transducer must also not inhibit the operation of the ultrasonic transducer. In other words, the electrical joint must have very little mass to affect the operation of the ultrasonic transducer. In addition, the electrical joint must not affect any of the operational characteristics of the ultrasonic transducer. The electrically connected ultrasonic transducers must be both mass producible and highly uniform in their operational characteristics.
Despite the inclusion of all of the aforesaid features, the system of the present invention shall utilize a minimum number of parts, all of which are of inexpensive construction, yet which afford the assembled ultrasonic transducer and connectors the high degree of precision and uniformity which must be retained. The design of the present invention must also enable it to compete economically with previously known designs, and it must provide an ease of accomplishment which is at least as high as competing designs. The design must accomplish all these objects in a manner which will retain and enhance all of the advantages of reliability, durability, and safety of operation. The system of the present invention must thus provide all of these advantages and overcome the limitations of the background art without incurring any relative disadvantage.